Polysaccharides are ubiquitous polymers in nature featuring highly complex molecular structures. In recent years, polysaccharides have emerged as important functional materials because of their versatile and unique properties such as biocompatibility, biodegradability and availability of reactive sites for chemical modifications to optimize their properties.
Polysaccharides are naturally occurring polymers that are composed of monosaccharides, linked through glycosidic bonds and are found in a large variety of natural resources such as microbes, plants and animal realms. Due to their wide range of molecular weights and different chemical compositions, polysaccharides exhibit diverse physiological, chemical and biological properties. Moreover, with other unique features such as low toxicity, biocompatibility, biodegradability and multivalent binding ability, polysaccharides become attractive biomaterials in the biomedical field (Mizrahy, 2012; Raemdonck, 2013).
The presence of various functional groups in polysaccharides allows their modifications and fine-tuning their properties for biomaterial applications in tissue engineering and regenerative medicine. Consequently, different methods have been developed for the modification of polysaccharides using their lateral functional groups (e.g. —OH, —COOH) to perform etherification, esterification, amidation, sulfation, free radical initiated co-polymerization and reductive amination.
The overwhelming majority of the methods to modify polysaccharides employ random chemical modifications, which in many cases improve certain properties while compromising others. On the other hand, the employed methods for selective modifications often require excess of coupling partners, long reaction times and are limited in their scope and wide applicability.
In addition, most of these methods deal with: 1) harsh reaction conditions to which some polysaccharides cannot tolerate, 2) pre-modification of the parent polysaccharide to install the requested reactive functional groups along the backbone, 3) non-selective modifications across the chain, which often change the structure of the resultant polymers and could significantly affect their physical and biological properties. For example, alginate hydrogels are widely used in tissue engineering as the ionically cross-linked hydrogel to maintain cell viability and function during the mild gelling process. To improve the properties of these hydrogels, alginate chains are modified, for example, with cell adhesion peptides such as arginine-glycine-aspartate (RGD), by employing carbodiimide chemistry to randomly modify the carboxylic groups in alginate monomers (Alsberg, 2001; Re'em, 2010; Shachar, 2011). As a result, this might reduce the availability of the carboxylic acid groups for calcium crosslinking, thus affecting the extent of alginate gelation and leading to relatively poor mechanical properties of the hydrogel.
Alginate is a polysaccharide derived from brown seaweed. It is an anionic polysaccharide composed of uronic acids (guluronic (G) and mannuronic (M) acids) that undergoes gelation in the presence of bivalent cations, such as Ca2+ and Ba2+. In the pharmaceutical/medicinal fields, it is used successfully as encapsulation material, mostly for cells (bacterial, plant and mammalian cells).
In order to overcome these difficulties, end group chemistry has been developed for the selective modification of polysaccharides, where the terminal carbonyl group undergoes modification without affecting the rest of the functional groups. This has attracted much attention because it preserves the inherent physical properties of the natural polysaccharide as well as obviating the need for pre-modifications (Schatz, 2010). Generally, this approach involves condensations via imine (Bosker, 2003; Guerry, 2013; Zhang, 2013) and oxime-forming reactions carried out on the terminal aldehyde (in equilibrium with hemiacetal).
Oxime chemistry has emerged as a powerful tool for the chemoselective conjugation of polysaccharides, owing to the high reactivity of the aminoxy functionality with the aldehyde group. In addition, this reaction can be performed in aqueous medium, under mild acidic conditions and the resulting oxime bond is stable at physiological conditions (Benediktsdottir, 2012; Novoa-Carballal, 2012; Styslinger, 2012). However, the overwhelming majority of the reports employing such a chemistry often use excess coupling partners, prolonged reaction times (>24 h) and low pH (WO 2003/024984), to which some polysaccharides may be sensitive as is the case with alginate, which is known to form gels at pH≤3 (Draget, 1994).
The urgent need of efficient bioconjugation chemistry to modify macromolecules (e.g. proteins) prompted the development of aniline and its derivatives as nucleophilic catalysts for rapid oxime formation. These catalysts enable the generation of a more populated protonated aniline Schiff base from the less populated carbonyl group and subsequent transimination with nucleophilic oxyamine (Dirksen, 2008; Dirksen, 2006; Rashidian, 2013; Wendeler, 2014).